In the mineral processing industry, comminution is the process by which solid materials are reduced in size, typically by crushing and then subsequent grinding processes, particularly to liberate valuable minerals from the mined material in which they are embedded. Comminution processes are also employed in various other industries, including cement, fertiliser, solid fuel, textile and pharmaceutical industries.
Grinding operations are commonly carried out in tumbling mills, which achieve size reduction of feed material particles by impact and attrition. Known forms of tumbling mills include:                ball mills, in which the feed material is ground by friction and impact with grinding media in the form of tumbling balls in a rotating cylindrical chamber;        autogenous mills, in which larger particles of the feed material itself replace the balls of a ball mill as the grinding media, and        semi-autogenous mills, which use larger particles of the feed material, aided by balls, as the grinding media.        
Autogenous and semi-autogenous tumbling mills typically reduce feed material particles from up to notionally 200 mm down to a product size of about 75 μm, whilst ball mills typically reduce feed material particles from up to notionally 15 mm to a product size of about 20 μm. These conventional tumbling mills are generally accepted to be energy inefficient processes. It has been estimated that the energy efficiency for these processes range from about 0.1% to 2%, based on the generation of new surface area. Operation of tumbling mills requires a substantial amount of energy to rotate the large cylindrical chambers filled with grinding media, feed material particles and slurry (created with the addition of process fluid to the chamber). Most of the input energy is dissipated in the form of heat and noise.
Another more recently adopted form of grinding is by way of high pressure grinding rolls, which compress a material bed of feed material particles between contra rotating rollers. High pressure grinding rolls have proved to be more energy efficient in reduction of feed material particle sizes from up to notionally 70 mm to a product size of about 4 mm. High pressure grinding rolls are reported to be 10% to 50% more energy efficient than tumbling mills, with less sensitivity to changes in feed material hardness. High pressure grinding rolls are, however, limited to dry grinding, with a maximum moisture content of about 10%. This limitation is caused by sliding friction on the rollers, whilst they draw feed material into the compression zone formed in the material bed. Specific compression pressure used between the rollers is typically within the range of 3 to 5 MPa. Micro-cracking of the feed particles benefit further downstream comminution, which is a further benefit of high pressure grinding rolls.